‘Green-beard’ genes could explain how altruism arose in nature

‘Green-beard’ genes could explain how altruism arose in nature

Sub: Science and tech

Sec: Health

Context: msp

• Scientists have gained valuable new insights into natural altruism by studying the amoeba Dictyostelium discoideum.

What is Altruism in animals?

• Altruism in animals describes a range of behaviors performed by animals that may be to their own disadvantage but which benefit others. 
• The costs and benefits are measured in terms of reproductive fitness, or expected number of offspring.
• Altruism is widespread in nature.
• Worker honey bees devote their entire life to foraging and caring for their sister, the queen, and her offspring, but do not themselves reproduce.
• In widow spiders, a male allows a female fertilized by him to eat him, and thus nourish herself and her offspring.
• A meerkat, a mongoose found in Africa, assumes the role of a sentinel, perching itself on a mound or rock, keeping a lookout for predators, instead of foraging for food, while the rest of the clan is feeding. If a predator is sighted, it alerts the others.

How can the emergence of altruism in all these diverse forms be explained?

• The question can be answered from the  studies of a simpler organism that has been easier for researchers to study: the social amoeba Dictyostelium discoideum. 
• Studies say that, if a gene makes a worker bee altruistic, it also helps the copy of the gene in the queen and her offspring to be passed on to the next generation, even if the worker herself does not reproduce.
• ‘Green-beard’ genes allow the individuals bearing them to recognise and preferentially cooperate with each other.
• A green-beard gene could provoke individuals to behave harmfully towards those carrying a different version of the gene.
• Green-beard genes encode some kind of tag that helps the genome to know their identity (i.e. self-recognition).

About Dictyostelium discoideum:

• Dictyostelium discoideum is a free-living, fast-growing, unicellular amoeba. In the wild, it feeds on bacteria that grow on decaying vegetation.
• Dictyostelium discoideum is a species of soil-dwelling amoeba belonging to the phylum Amoebozoa, infraphylum Mycetozoa.
• Commonly referred to as slime mold, D. discoideum is a eukaryote that transitions from a collection of unicellular amoebae into a multicellular slug and then into a fruiting body within its lifetime.
• About 20% of the amoebae in an aggregate altruistically sacrifice themselves to form the stalk. 
• The remaining 80% become the spores.

How does D. discoideum ensure that cheaters do not prosper?

• Two genes in the D. discoideum genome called tgrB1 and tgrC1, displayed all the properties one would expect in a green-beard gene.
• The tgrB1 and tgrC1 genes are located next to each other in the D. discoideum genome, and are expressed together
• They contain information for cells to make two cell surface proteins called TgrB1 and TgrC1. 
• The TgrB1 protein on one cell binds to the TgrC1 protein on another.  
• If the binding is strong, the TgrB1 protein is activated, and confers altruistic behavior – manifesting as the amoeba’s willingness to form the stalk.
• The binding between the TgrB1 and the TgrC1 proteins of cells of the same strain is strong, and leads to self-recognition and cell-cell cooperation. 
• The tgrB1 and tgrC1 genes are also very polymorphic: within the same population of D. discoideum amoebae, they have multiple variants. 
• The researchers were able to correlate differences in the tgr gene sequences between two strains to the efficiency with which their cells segregated from each other in mixed aggregates and formed separate fruiting bodies.
• When the binding of TgrB1 and TgrC1 proteins across the cells of diverged strains was weak, TgrB1 failed to be activated, and the cells split away from each other instead of cooperating.
• When the researchers deleted the tgrB1 gene but left the tgrC1 gene intact, the amoeba did not cheat on ‘non-self’ amoebae that carried a different tgrC1.
• Instead, it cheated those with the same tgrC1 variant as itself — i.e. its kin. Every family has its black sheep!